The option of the appropriate shading device depends on the latitude, sky conditions (the direct, diffuse and reflected solar radiation components) orientation, building type and overall design of the building (Santamouris & Asimakopolous, 1996). Such devices can be used to decrease heat gain through the day and the heat loss through the night, as long as they are of appropriate design and orientation (O'Cofaigh, Owen, & Fitzgerald, 1999).
Orientation of the windows, combined with their size, can decrease the solar gains passing through them in the following ways (Santamouris & Asimakopolous, 1996):
1. North oriented windows get limited solar heat gains (at early morning and late afternoon hours during summer).
2. South oriented windows get high solar heat gains during winter, while they can simply be shaded during summer due to the high position of the sun in the sky. 3. Siting of east and west oriented openings is a problem due to the low sun
position in the sky. It is advisable that openings in this side should be as small as possible or exchange with other orientated openings in these facades.
4. Skylights present problems in shading because they face the sun directly.
Shading devices may be fixed or movable or retractable and may be placed externally, internally or within double glazed panels. Vegetation can also be considered as an external shading element.
External shading devices
Windows completely shaded from the outer surface decrease solar heat gains by up to 80% (ASHRAE, 1993). Once infra-red radiation has entered through the glazing into
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the building, the majority of it is captured and has to be dissipated by ventilation or air conditioning units (McNicholl & Lewis, 1994). In which they are additionally expensive to fit and present a problem in their repair but they play a significant role in visual character on elevation (O'Cofaigh, Owen, & Fitzgerald, 1999).
Adjustable external shading devices include shutters (hinged, sliding, etc) rotatable fins, horizontal plates, retractable venetian blinds, or canvas awnings. They can be made of various materials, such as wood, different metals or fibre. Many exterior shading devices can cut off solar radiation reflected from the floor, as well as intercepting the direct and most of the reflected radiation from the sky, as shown in Table 2.1 (Givoni, 1994).
External shading devices can be of three basic type’s figure 2.24:
1. Vertical devices: these consist of aperture blades or fins in a vertical position. 2. Horizontal devices: these could be canopies, horizontal louver blades or
externally applied venetian blinds.
3. Egg crate devices: these include various types of grill blocks and decorative screens.
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Figure 2.24 Solar radiation transmission through external shading (O'Cofaigh, Owen, &
Fitzgerald, 1999).
Table 2.1 Solar radiation transmission through external shading.
Colour % Transmission % Reflection % Absorption Light-coloured,
translucent
25 60 15
White, opaque 0 80 20
Dark, opaque 0 12 88
Source: (Goulding, Owen, Steemers, & Directora, 1992)
1. Fixed or movable overhangs and canopies
These devices are used to shade south-facing openings, and are positioned on top of the glazed element. It is important to place the overhang in such a location that it facilitates the passing of the rays through the opening when the sun is low in the sky as can be seen in Figure 2.25. The depth of the overhang must take into account its distance over
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the window as well as the opening height, although its length is determined by the opening width (Lewis, Goulding, & Steemers, 1992).
Figure 2.25 Overhang angle
These devices block high angle sunlight and allow the low angle winter sun, although they also reduce daylight diffusion (Stack, Goulding, & Lewis, 1999); for that reason awnings or ductile hangings can be best in northern latitudes (McNicholl & Lewis, 1994). Continuous overhangs provide, to a large extent, more shade than those across the width of the opening only (O'Cofaigh, Owen, & Fitzgerald, 1999).
Long balconies and roof overhang are more suitable in hot climates: in many cases this is accomplished with tents or pergolas as shown in Figure 2.26 (Santamouris & Asimakopolous, 1996).
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Figure 2.26 Sun path on summer and winter latitude.
2. Light shelves
Light shelves are considered successful devices for the eradication of direct sunlight in the front of the room without dropping the daylight aspect at the back of the room (O'Cofaigh, Owen, & Fitzgerald, 1999). They divide the purpose of the opening horizontally: a bottom part protected by an overhang to illuminate the front part of the room, and a higher part gives illumination for the back part of the room. In a strongly sunny climate, the situation might need an additional overhang on the upper window to avoid direct sunlight from getting to the back of the room (Baker & Steemers, 2002).
3. Fixed and movable louvres
Louvres are the best on the east and west facade. Moreover, horizontal louvres have to be almost totally closed to stop sunlight on east and west elevations. Vertical louvres can be left partly open to allow reflected or spread sunlight from the north, while still blocking direct sunlight (Goulding, Owen, Steemers, & Directora, 1992).
Louvres must be located so as to obstruct the summer sun from shining towards the inside of the building and at the same time allow the access of the winter sun. They also function as a light diffusing device during the summer (when painted white) by diffusing the light into the room below (Santamouris & Asimakopolous, 1996).
4. Shutters
Shutters can be louvered or opaque. Their shading performance changes according to their colour. White shutters are better than dark ones, as they give additional daylight by reflecting the solar radiation inside.
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Opaque shutters are classified an efficient tool for daylight control. However, louvered shutters merge the advantages of retractable non-transparent shutters and the light redistributing characteristics of louvers when closed. They can be adjusted to stop direct sunlight permeation and only allow the ground reflected light to go through (Baker & Steemers, 2002).
5. Fixed screens
These devices can be found in tropical buildings, where screens are used in unpainted slots. They do not only offer shade to the window, but they also maintain safety and privacy from outside, at the same time as allowing ventilation (Baker & Steemers, 2002). If the screen is intended to be used in shading the east and west openings, this can determine the dimensions of the width and height of the opening. (Lewis, Goulding,
& Steemers, 1992).
6. Egg-crate
The egg-crate shading device combination is the most efficient in the south-east and south-west facades. These are considered as successful for east and west orientations in hot climates and for south-west facades in very hot climates (Santamouris & Asimakopolous, 1996).
7. Awnings
Heat gain can be reduced by 65% in summer on a southern elevation and by 80% on western elevation by adding awnings to the building, as shown in Table 2.2. The required dimensions of awnings are similar to those for horizontal overhangs, but their effectiveness depends on how opaque the material is, as well as the presence of dirt and dust on them, which might vary their absorption characteristics. The effectiveness of material awnings is likely to get worse with age or weather damage (Goulding, Owen, Steemers, & Directora, 1992).
Table 2.2 Solar transmittance of awning materials.
Material % Direct Transmittance % Diffuse Transmittance
Canvas 0 0
Plastic 25 15
Aluminium (Separated slats)
0 20
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Internal shading devices
In general, interior shading devices can by simply and easily adjusted and are also able to be used for providing privacy (McNicholl & Lewis, 1994). They can be of several designs, including curtains, roller blinds or others. However, they are less efficient than external shading devices as they reduce only a small portion of the solar radiation which has previously penetrated and been reflected at their surface and transmitted from the exterior through the glazing. The remaining portion of the radiation is absorbed and radiated to the room. In addition, they conflict with day lighting and natural ventilation flowing into the building (Santamouris & Asimakopolous, 1996). Therefore, internal shading devices are unlikely to be suitable for hot regions especially when applied to a large opening (Givoni, 1994). The reflection and absorption properties of an internal shading device influence the amount of solar radiation passing into the building (Goulding, Owen, Steemers, & Directora, 1992). Although it is known that white shading devices reradiate more radiation to the outside compared with dark ones, in the general heat gain of any shading device inside the window is much greater than that of an external shading device (Givoni, 1994).
1. Fabric blinds and curtains
Fabric blinds differ in reflectance and transmittance, which has a significant result on their shading/day lighting purpose (Baker & Steemers, 2002). The colour of the blind or curtain determines its shading effectiveness. Light coloured blinds are more effective than those which are dark coloured, because of the effect shown in Figures 2.27 and 2.28 (Santamouris & Asimakopolous, 1996). A study has shown that light coloured internal curtains give as much as 18% additional shade protection compared with by darker ones. It has been shown that an aluminium blind can add an extra 10% more shielding than a coloured one (Olgyay, 1963).
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Figure 2.27 Effect of dark coloured blinds.
Figure 2.28 Effects of light coloured blind.
Fabric blinds and curtains are not considered as very efficient shading devices because they stop the passage of radiation; they thus absorb the solar heat and can get to an extremely high temperature. Some part of the heat absorbed will be transmitted inside the room air and the rest will be re-radiated. The gap between the window and the blind or the curtain can become very hot (Koenigsberger, Ingersoll, Mayhew, & Szokolay,
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1973), therefore glass with absorptive and low emissions must not be used with internal blinds (Goulding, Owen, Steemers, & Directora, 1992). This effect can be reduced by 15-20% with the use of reflective blinds (O'Cofaigh, Owen, & Fitzgerald, 1999).
2. Venetian blinds
Horizontal venetian blinds can be efficient devices in shading the zone close to the window and at the same time they reflect light on to the ceiling and thus to the back of the space (McNicholl & Lewis, 1994) to reduce discomfort due to direct sunlight and permit more penetration of daylight (Table 2.3), although they tend to collect extra dust. Vertical blinds present better light control and have fewer maintenance problems. (Crowther, 1992). For their angular control, raising and lowering are achieved both by hand and by mechanical control. For most effective results occupants must have information on how to make use of such blinds (O'Cofaigh, Owen, & Fitzgerald, 1999). The effectiveness of shading depends on the colour and material of venetian blinds: an additional 20% of shade protection can be provided by selecting off-white venetian blinds (Santamouris & Asimakopolous, 1996).
Table 2.3 Transmission, reflection and absorption characteristics of venetian blinds (for
blind at 45° tilt with sunlight perpendicular to the slats.
Colour Type Transmission Reflection Absorption Light coloured Horizontal 5% 55% 40%
Medium coloured Horizontal 5% 35% 60%
White Vertical 0% 77% 23%
(Goulding, Owen, Steemers, & Directora, 1992)
3. Other internal shading devices
Light shelves as well as shutters can be internal, in addition to external shading devices. The louvres are able to be used fixed or rotating internally. However, an inner louvre is not as efficient at reducing solar heat as outer or even mid-pane types (Littlefair P. J., 1999).
Retractable shading devices
“Retractable” shading devices can be totally or partly separated from the window opening. They can take the form of retractable blinds and curtains, as shown in Figure 2.29, and may be either shutters or louvres. It is significant to note that shading devices
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of this kind have only a small amount of influence on the availability of daylight in the space, for the reason that they can be removed at periods of low light accessibility (Baker & Steemers, 2002).
Figure 2.29 Reducing solar heat gain by various types of shading devices (Stack, Goulding,
& Lewis, 1999)
Fixed shading devices
Fixed shading devices are a common architectural characteristic. Their proposals should take into account the orientation of the opening to be protected. Their efficiency changes in accordance to the seasonal changes in the situation of the sun (Lewis, Goulding, & Steemers, 1992). On account of this factor, the common use for horizontal fixed shading devices is in southern facades, while east and west facing facades can benefit from sideways shading (Goulding, Owen, Steemers, & Directora, 1992). Vertical fins protect northern openings in the morning and late afternoon hours from the low sun in summer (Givoni, 1994).
Fixed shading includes both structural elements, such as balconies projecting fine or shelves, and non-structural elements such as canopies, louvres and screens. These are commonly used on the exterior facades where they can stop direct solar radiation from reaching the windows and dissipate the absorbed heat to the outside air (Goulding, Owen, Steemers, & Directora, 1992). Occupants of the building choose to install these aids to cooling because of their simplicity, as well as their low maintenance and construction costs (Santamouris & Asimakopolous, 1996).
Mid-pane shading devices
Mid-pane shading devices have been developed to deal with the issue of interference with both views and ventilation, which accompanies the shading effectiveness of mainly
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retractable blinds and curtains. They can operate either manually or automatically (Baker & Steemers, 2002). There are several kinds of mid pane devices:
1. Mid-pane venetian blinds
Different types of mid-pane blinds are in use. They include blinds with in the air gap of double windows or double skin facades (there can be a gap as wide as a metre between the internal and the external walls of some modern buildings). They generally consist of micro venetian blinds, which can be operated by magnets or by a tiny electric motor. They are more advantageous than internal blinds but difficulty of access can be a problem (Littlefair P. J., 1999). A few types of blind systems use absorptive and reflective specula transmitting films (Baker & Steemers, 2002).
2. Mid-pane fixed reflective louvres
This style of mid-pane device is highly recommended for openings that receive high solar heat gain (especially horizontal or sloping roof lighting). They do not cause much of a problem in their maintenance when they are installed inside double glazing units. The major issue with this fixed kind is they deal efficiently with the sun only for a limited range of solar altitudes (Littlefair P. J., 1999).
3. Mid-pane cloud gel
This gel is sealed between double glazing panes and is normally clear. Above about 30oC, it becomes milky white and opaque, therefore excluding unwanted radiation (Littler & Thomas, 1984).
Reducing window area
Reducing the area of the window is suggested as a strategy for windows which receive a great amount of additional solar heat gain and where loss of day light is not an issue. The disadvantage of smaller windows is the loss of view. The British standard on day lighting (British Standards Institute, 1999) recommends a minimum view window area of at least 20% of the inside window wall, and larger than this for extremely deep areas (Littlefair P. J., 1999).
Comfort windows can provide a similar effect to vertical fins as long as the glazing is close to the inside of the window wall (Littlefair P. J., 1999).